Automatic gain control circuit



April 13, 1937. H, BA EL T A 2,077,139

AUTOMATIC GAIN CONTROL CIRCUIT Filed May 12, 1954 3 Sheets-Sheet l flvpur 007F07- F i l l l INVENTORS //A/V6 B/I/PTELS ATTORN EY April 13, 1937. H. BARTELS ET AL AUTOMATIC GAIN CONTROL CIRCUIT :s' Sheets-Sheet :5

Filed May 12, 1934 INVENTORS #4: BARR-LS HANS F R/Ch BY VAR/ABLE C 0/1 7' 301 V0! 71465 ATTo'RNEY Patented Apr. 13, 1937 UNITED STATES AUTOMATIC GAIN CONTROL CIRCUIT.

Application May 12, 1934, Serial No. 725,264 In Germany April 12, 1933 Claims.

The present invention is concerned with improvements in automatically regulated amplifier apparatus. It is known that automatically regulated amplifier arrangements are employed for 5 the purpose to reduce the large dynamic power of original programs or productions, in the case of the interposition of electrical transmission systems, to the value which can be transmitted by such transmission systems.

In devices serving to insure automatic gain .control, as is well known in the art, transmission means are inserted in the amplifier path whose transmission measure is variable as a function of a biasing voltage. However, the variation of the amplification factor or gain, i. e., of a biasing potential, is associated with a change of the static current of the regulator means furnished from a direct current source such as, for instance, an electronic tube or a rectifier. For example, in the case of an electron tube of the kind used for anti-fading or fading compensation purposes, a large plate feed current (static current) corresponds to high gains, and small feed currents to low gains.

In order to preclude feedbacks by way of joint sources of current, and to prevent supply-line noises or hums in mains-connected amplifiers, it is necessary in the case of amplifying devices to include filter networks in the plate leads of the tubes. If in a similar way a gain-control apparatus is connected with the same batteries, or with, a power pack united with a supply-line, plate filters must be provided also in this case so as to prevent feedbacks and to eliminate disturbing frequencies.

' It has also been found that automatic gain control must be effected inside a very brief length of time, in fact, the building-up period should preferably be less than 100 milli-seconds. However, since the building-up times of the filters are considerably greater if sufficient plate smoothing is to be insured (the same ranging around 1 sec. and over), difficulties are encountered in the attempt of combining plate filter networks in connection with automatic gain or volume control schemes. Now, the purpose of this invention is to overcome difficulties in the said direction.

Inasmuch as in normal circuit schemes, as above pointed out, plate filters can not be used,

50 the regulator tubes according to the present invention, may be provided with a distinct D. C. voltage (or separate auxiliary grid voltages). Owing to the low internal resistance of the plate battery, and in the absence of plate filters, the

55 building-up and the decay period and constant of the regulator system can be made in practice of any desired small value.

Another solution would reside in the use of PATENT OFF-ICE.

artificial networks which havethe purpose of making conditions so that, in the presenceof variations of the feed'current of the regulator system, the current derived from the, D. C. source will remain constant or practically so.

It is also feasible to employ multi-grid'tubes, say tubes with a space-charge grid (or spacecharge tubes), in which thestability or constancy of the emission current (i. e., of the platecurrent plus space-charge current) is used for the purpose of stabilizing the value of the aggregate current from-theD. C. source conducted by way of the filter chain,in spite of changes in the plate feed current of this tube. Instead of a space-charge tube, it would also be feasible to use tubes with several auxiliary grids for the purpose of insuring the identical effect.

The novel features which we believe to be characteristic of our invention are set forth in particularity inthe appended claims. The invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description, taken in connection with the drawings, in which we have indicated diagrammatically several circuit organizations whereby our invention maybe carried into effect. 1

In the drawings:-- 7

Fig. 1 shows a push-pull circuit embodying one form of the invention,

Fig. 2 shows a modification of the arrangement in Fig. 1,

Fig. 3 illustrates another embodiment of the invention,

Fig. 4 shows a modified form of the arrangement shown in'Fig. 3, v

Fig. 5 shows still another embodiment of the invention. v

Figure 1 shows asection from the circuit diagram of an ordinary push-pull amplifier, to which the invention has been applied, this figure representing only the part of the automatic arrangement for the control of the amplification. The electrical impulses are supplied from the input side of the transformer l, connected as mentioned, in push-pull, from where the impulses are passed to the gridsof the controlled tubes'Z and. 3. The grid bias voltage source 4 may be. of any type. The heating circuits for the tubes are omitted. from the figure. Furthermore, the impulses are supplied to the transformer 5. The

anode potential for the controlled tubes 2 and 3 is taken from a rectifier, not shown, across a filter chain 5. The time constant of this filter chain 6 is of such value that at any variations of the anode current for the purpose of regulating the amplification, the amplification changes occur practically only after a greater length of time. I

According to the invention, an ohmic resistor I is inserted in the anode circuit, namely between filter chain 6 and controlled tubes 2 and 3; the current received by this resistor is many times the anode current received by the controlled tubes. It has been found advantageous to make the current in the resistor 1 approximately ten times as large as that for the anode circuit of the controlled tubes. Due to this resistor the anode filter chain 8 is damped to such an extent that the desired variation in the anode current of the control tubes is made possible within the time value last referred to. The time constant of the filter chain is only of minor importance in view of the small part of the anode current variations as compared with the total current.

Figure 2 shows a circuit, in accordance with Figure 1, whereby the ohmic resistor 1 in Figure 1 is replaced by a glow tube 8. The functioning of this circuit is identical with that of the circuit according to Figure 1.

Figure 3 shows a control circuit for hexodes, in which the hexodes are connected for instance in push-pull. The current impulses from the input side are supplied to the grids 2 and I3 of controlled tubes In and across the transformer 9. The grid battery l4 may consist of any type of current source furnishing a negative potential. The heating circuits for thetubes is likewise not shown in particular. The second grids l5 and N5 of the tubes are placed at a positive potential across battery H. The further grids l8 and I9 are connected to the plate current source 20, the latter having also connected thereto the anodes 2| and 22 of tubes-l8 and ||,across the pushpull connected primary windings 23 and 24 of output transformer 25. The output circuit is placed at the secondary windings 26 and 21 of the transformer. The grids 28 and 29 of tubes I9 and II are connected to the negative voltage source 3|, across a resistor 38. Ahead of the input transformer 9 the incoming current impulses are in part branched off through lines 32 and 33 in parallel to said transformer, and they are ap plied to the resistor 38 through a path including an amplifier 34, rectifier 35 and a time constant unit 36.

In the tube l8 the following occurs, and the same is likewise true for the tube The electrons emitted by the cathode of the tube are in fluenced at the frequency of the impulses impressed upon the grid l2 by the transformer 9, and they are passed to the grid l5 which, due to its positive potential, acts as an anode. The voltage of this grid acting as an anode, is equal to, or lower than, the potential at anode 2|. A portion of the electrons passes through grid l5, this passage being dependent upon the throughgrip (reciprocal of amplification factor) of grid l5, and furthermore these electrons reach grid I 8. While a part of the remaining electrons is deviated by I8, a further part of the electrons reaches the control grid 28, and then the anode 2|. The sum of the electrons received by the grid l8 acting as anode, and by the anode 2|, is constant, and hence this is also true for the total current flowing in the circuit (battery ll/Zll and tube l8), containing the parallel connected grid l8 and anode 2|. The current distribution in the current branches placed at grid l8 and anode 2|, however, depends upon the voltage condition of grid 28, namely in such manner that when the negative voltage at the control grid 28 decreases, i. e. towards the positive, the current in the branch placed at the anode 2|, increases, and the current in the branch at |8 decreases. When the negative potential at the control grid 28 becomes higher, 1. e. towards the negative, the reverse takes place. 4

In the circuit, according to Figure 3, the impulses for the control grids 28 and 29 are derived from a place ahead of the input transformer 9 of the control circuit. In Figure 4 the same control circuit is shown; however, in this case, the impulses for the control grids 28 and 29 are tapped in back of the output transformer 25 preferably behind a further amplifier 31. In the circuit according to Figures 3 and 4 corresponding parts are designated by identical reference characters. The danger of feed back is less in a circuit according to Figure 3 than in the case of the circuit of Figure 4. The mode of control is obviously not solely limited to the push-pull circuit.

In the circuit according to Figure 5 a further example for a control arrangement is shown for instance for a simple pentode circuit. In the circuit of Figure 5, the impulses are applied to a pentode across the input transformer 38 and grid 39 of the tube 40. The grid 39 receives a negative potential from the voltage source 4|. The grid 42 acting as an anode is placed at voltage source 43, to which there is also connected the anode 44 across the primary winding 45 of the output transformer 46. The output circuit is connected to the secondary winding 41 of transformer 46. The control grid 48 of the control tube 48 is-connected to a negative voltage source 49. The

variable control voltage is applied to the ter minals 58. The connection ofthe control voltage is carried out in the manner shown in the diagrams of Figure 3 or 4. The control performance is similar to that indicated for Figures 3 and 4. The electrons are influenced at the frequency of the impulses impressed upon grid 39. portion of the electrons deviated by grid 42 acting as anode, and by the anode 44, i. e., the currents flowing through the circuits connected to these electrodes, will be influenced by the variation of the control voltage at grid 48, in the manner above described.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What is claimed is:

1. In combination with a source of electrical Waves and a load output circuit, a pair of electron discharge tubes, each of said tubes including a cathode, an input grid, an output anode, a positive grid disposed adjacent said input grid, and a gain control grid disposed between the anode and said positive grid, the input electrodes of said tubes being arranged in push-pull relation, and the output anodes of said tubes being arranged in similar push-pull relation, means for deriving from the electrical waves a direct current voltage whose magnitude is dependent upon the magnitude of said waves, and means for impressing said direct current voltage upon the gain control grids The proof said tubes in a sense such that the gain of said tubes is inversely related to the amplitude of said waves.

2. In a system as defined in claim 1, each of said tubes including in addition a positive grid disposed between the input grid and the first mentioned positive grid.

3. In a system as defined in claim 1, said means for producing said direct current voltage comprising a network which includes at least a rectifier, a time constant network and an impedance for developing said direct current voltage.

4. In a system as defined in claim 1, said impressing means including a source of fixed direct current voltage for maintaining a normal negative bias on said gain control grids of both tubes in the absence of incoming electrical waves.

5. In combination with at least one tube having a cathode, a positive plate, a normally negative signal grid adjacent the cathode, a gain control grid adjacent the plate, a positive grid disposed between the signal and gain control grids, said cathode, signal grid, positive grid, gain control grid and plate being arranged in the order named, an audio current source coupled between the cathode and signal grid, an audio output circuit coupled to the plate, a source of direct current voltage having its negative terminal connected to said cathode and its positive terminal connected both to said plate and positive grid, a gain control network comprising an amplifier having its input connected to the said audio source, a rectifier coupled to the amplifier, a time constant circuit connected to the rectifier output, and means for impressing the rectified, audio voltage upon the said gain control grid.

HANS BARTELS.

HANS FRIEDRICH. 

